The present invention relates to a reformer reactor for converting hydrocarbon fuels into hydrogen rich gas for fuel cells and/or exhaust treatment applications by auto-thermal reaction and a method for converting hydrocarbon fuels into hydrogen rich gas using the reformer reactor.
In the state of the art it is well known to produce hydrogen rich gas for the use in fuels cells of transportation devices by reforming hydrocarbon fuels, like gasoline or diesel fuels. Conventionally, hydrogen is produced in large-scale industrial facilities and then stored on board of the transportation devices. The recent development of small-scale on-board hydrogen sources, so-called reformer reactors, provides a possibility for producing hydrogen on demand without the necessity of hydrogen storage.
In general there are three known methods of reforming gaseous or liquid hydrocarbon fuels into hydrogen: catalytic steam reforming, partial oxidation reforming and auto-thermal reforming.
In catalytic steam reforming processes a mixture of steam and hydrocarbon fuel is exposed to a suitable catalyst, like nickel, at a high temperature (between 7000 C and 1000° C.). The reaction is highly endothermic and requires an external source of heat and a source of steam.
In partial oxidation reforming processes a mixture hydrogen fuel and an oxygen containing gas, like ambient air, are brought together within a reaction chamber and subjected to an elevated temperature, preferably in the presence of a catalyst. The catalyst used is normally a noble metal or nickel and the temperature is between 700° C. and 1700° C. The reaction is highly exothermic and once started generates sufficient heat to be self sustaining. In order to promote the oxidation reaction it is necessary to preheat the feed fuel and to reduce temperature variations in the reactor.
U.S. Pat. No. 6,770,106, for example describes a partial oxidizing reformer for reforming feed gas containing hydrocarbon or methane wherein the reduction of temperature variations is achieved by an reactor being covered with a passage for preheated feed gas and therefore being thermally isolated by the feed gas passage. The reaction heat is recovered by a heat exchanger for the purpose of preheating the feed gas.
Auto-thermal reforming processes are a combination of steam reforming and partial oxidation reforming. Waste heat from the partial oxidation reforming reaction is used to heat the endothermic steam reforming reaction.
The natural by-products of all reforming processes are carbon monoxide and carbon dioxides. But, since the hydrocarbon fuels were not designed as a feed stock for generating hydrogen, there are also other by-products such as sulphur, olefins, benzene, methyl amid and higher molecular weight aromatics. These by-products may be harmful to the fuel cells and should therefore be removed by subsequent steps outside the reformer reactor.
Another disadvantage is that hydrocarbon fuels, especially diesel, tend to burn to soot by contact with the inside surfaces of the reactor. Soot particles again, are very harmful to the fuel cells and have to be removed before the reformed hydrogen is applied to the fuel cells.
It is desirable to reduce the risk of fuel molecules burning to soot by coming into contact with the inside surfaces of the reactor.
According to an aspect of the present invention the inner wall and therefore the inside surfaces of a reaction space is charged with a first electric charge. This takes into account that the fuel molecules sprayed into the reaction chamber by a fuel inlet are charged. Charging the inner walls of the reactor housing with a certain electric charge statistically prevents 50% of the fuel molecules to come into contact with the inside surfaces of the reaction chamber because of electrostatic repulsion. This reduces the risk of fuel molecules burning to soot, significantly.
According to another advantageous embodiment also the fuel inlet is charged. Preferably, the fuel inlet is charged with equal electric charge as the inner wall. This results in that all fuel molecules have the same electric charge as the inner wall and are therefore electrically repelled there from. Thus, it is almost impossible that the fuel molecules come into contact with the inner wall and burn to soot.
Another preferred embodiment is provided with a catalyst for the auto-thermal reaction inside the reaction chamber to accelerate the conversion of hydrocarbon fuel into hydrogen rich gas. Preferably, the catalyst can also be charged. This is realised in another embodiment, wherein the electric charge of the catalyst is opposite to the electric charge of the inner wall and/or the fuel inlet. This accelerates the fuel/oxidising agent mixture toward the catalyst and consequently accelerates the conversion reaction. The catalyst can be a ceramic monolith or metal grid.
A further preferred embodiment of the present invention takes advantage of the fact that formation of soot by burning of fuel particles in contact with the inner walls of the reactor occurs only above a certain wall temperature. Remains the wall temperature below, for example by cooling the walls or by keeping the temperature of the reaction space below a certain level, the burning of fuel particles to soot is prevented.
Generally, there is the possibility to provide the reactor with an external cooling device but this increases the dimension of the reactor and adds a further consumer of energy to the system being supplied with energy by the fuel cells.
Therefore, the preferred embodiment uses the relatively cool oxidising agent for cooling the inner wall of the reactor. That means at the same time that a thermal isolation of the inner wall can be left out, whereby the dimension of the reactor is further reduced.
Another advantage of the cooling of the inner wall is that the temperature inside the reaction chamber can be held constant and the temperature of the oxidising agent can be controlled.
As shown in another preferred embodiment of the present invention, the oxidising agent outlet provided in the inner wall of the housing is formed as a plurality of holes. This facilitates the homogenous distribution of oxidising agent in the reaction chamber. Preferably, size, shape and/or location of the holes can vary according to the used oxidising agent, the used hydrocarbon fuel and/or their temperature. Most preferably, the oxidising agent outlet is provided in the vicinity of the fuel inlet.